(a) Field of the Invention
The present invention relates generally to a liquid crystal display device and a method and apparatus for driving the same, and more particularly to a liquid crystal display device and a method for driving the same for fast transition into bend state at initial operation such as immediately after power is inputted in a liquid display device with an OCB mode.
(b) Description of the Related Art
In general, as a liquid crystal display device is even thinner and lighter and consumes less power than a cathode ray tube (CRT) dominated up to now in the field of display device, now it is widely used as a display device in potable information apparatuses such as a mobile telephone and a notebook computer. In addition, as it has a weak emission of electromagnetic wave, it is also expected to be a mainstream in a display device for desktop in preference to the CRT in the future.
However, such a liquid crystal display device has a disadvantage in that a view angle property by which brightness and color change depending on a viewing direction is poor. Various approaches for addressing the disadvantage have been suggested.
For example, an approach for improving straightforwardness of light incoming from a backlight by use of a prism attached to a surface of a light plate so that brightness in a vertical direction is increased by more than 30% has been put to practical use and an approach for increasing a view angle by use of a negative light compensation plate is in application.
In addition, though an In Plane Switching mode has been developed to accomplish a wide view angle of 160° of about the same level as CRT, this mode needs further improvement due to its relative low aperture ratio.
Besides, many attempts have been made to increase the view angle, including approaches such as an OCB (Optical Compensated Birefringency) mode, a PDLC (Polymer Dispersed Liquid Crystal) mode, a DHF (Deformed Helix Ferroelectric) driven by TFTs.
Particularly, the OCB mode is now under lively development because of its high speed of response and its wide view angle.
Now, operation of the above OCB mode will be described with reference to FIG. 1.
FIG. 1 shows a view for illustrating operation of a typical OCB mode and FIG. 2 shows a view for ON/OFF cycle of the OCB mode.
Referring to FIG. 1, an initial alignment state of liquid crystal positioned between an upper electrode and a lower electrode is Homogenous state (referred to “H” state hereinafter). At that time, when a predetermined level of voltage is applied to the upper/lower electrodes, “H” state is transferred into Bend state (referred to “B” state hereinafter) via Transient splay (referred to “T” state hereinafter) and Asymmetric splay (referred to “A” state hereinafter) to operate as the OCB mode.
As shown in FIG. 1, a liquid crystal cell in the OCB mode is prepared by rubbing alignment films in same direction, with a pretilt angle in the vicinity of alignment films of about 5°–20° and a cell thickness of 4–7 μm. As orientation of liquid crystal molecules in the middle of liquid crystal layer becomes symmetric, a tilt angle becomes 0° below a specified voltage and 90° above the specified voltage. After the tilt angle of liquid crystal molecules in the middle of liquid crystal layer became 90° by applying a high voltage equal to or more than the specified voltage initially, polarization of light passing the liquid crystal layer is modulated by a change of tilt angle of the remaining liquid crystal molecules other than those in the vicinity of the alignment films and those in the middle of liquid crystal layer with the applied voltage varied.
In the above OCB mode, though it takes several seconds to change the tilt angle of the middle liquid crystal molecules from 0° to 90°, back-flow is not present for a variation of voltage after that and a response time is very short to be in order of 10 ms because of the bend state of the liquid crystal layer and a large modulus of elasticity.
As shown in FIG. 2, for ON state of the typical OCB mode, though a conversion from “T” state to “A” state is fast and a conversion from “T” state to “B” state is relatively fast, a conversion from “A” state to “B” state is slow. As also shown in FIG. 2, for OFF state of the typical OCB mode, though a conversion from “B” state to “H” state is slow, a conversion from “T” state to “H” state or from “A” state to “H” state is fast.
As is described above, the problems are present that it takes several seconds to obtain bend state for the OCB mode and, particularly, the liquid crystal of OCB mode can be worked only when bend state transition is induced through an entire liquid crystal panel by applying a high voltage during a short period after a power switch of a PC monitor or a TV turns on.
Accordingly, it is desirable to provide a technique for shortening time for obtaining bend state and reducing a voltage applied to accomplish bend state.